以地图为参数的功能不起作用

Question

For some reason, all of the functions in my program with a map as a parameter are not working. This function is the one that calls all of them (pageAndTimestamp is a struct btw):

void fifo(int framesize, int numref, int* pagestream)
{
  double hit = 0, size = numref;

  map<int, pageAndTimestamp> frames = frameMaker(framesize);

  for (int time = 0; time < numref; time++)
  {
    if (pageLoaded(pagestream[time], frames))
    {
      hit++;
      output(time, pagestream[time], size, hit, frames);
    }
    else
    {
      int loc = findPageToReplace(frames);
      replacePage(loc, pagestream[time], time, frames);
      output(time, pagestream[time], size, hit, frames);
    }
  }
}

These are the functions that are not working properly:

bool pageLoaded(int page, map<int, pageAndTimestamp> m)
{
  for (const auto& it : m)
  {
    if (it.second.a[0] == page)
    return true;
  }
  return false;
} 

int findPageToReplace(map<int, pageAndTimestamp> m)
{
  int timestamp = INT_MAX;
  int replaceLoc = 0;
  for (const auto& it : m)
  {
    if (it.second.a[1] == -1)
    return it.first;
    else
    {
      if (it.second.a[1] < timestamp)
      {
        timestamp = it.second.a[1];
        replaceLoc = it.first;
      }
    }
  }
  return replaceLoc;
}

void replacePage(int loc, int page, int time, map<int, pageAndTimestamp> m)
{
  m.at(loc).a[0] = page;
  m.at(loc).a[1] = time;
} 

void output(int t, int p, double s, double h, map<int, pageAndTimestamp> m)
{
  cout << "Time: " << t << endl << "Page: " << p << endl;
  for(const auto& it : m)
    cout << "Frame" << it.first << ": " << it.second.a[0] << endl;
  cout << "Hit ratio: " << h << " / " << s << " (" << h / s << ")" << endl 
 << endl << endl;
}

When I run the program in the Visual Studio 2017 debugger, when I step into any of the above functions, the debugger takes me to this function header in the map standard header:

map(const map& _Right)
    : _Mybase(_Right, _Alnode_traits::select_on_container_copy_construction(_Right._Getal()))
    {   // construct map by copying _Right
    }

I don't know what the problem is, or why the debugger is taking me to this function header. How do I fix this?

Answer 1

The function replacePage for example, is defined as:

void replacePage(int loc, int page, int time, map<int, pageAndTimestamp> m)

This function takes a map as a value , not a reference or pointer. Thus, when you call it as follows:

replacePage(loc, pagestream[time], time, frames);

then the map frames is copied into the variable m in your function. This is why the debugger, for example, takes you to the copy constructor for a map.

Further, it means that the replacePage code

  m.at(loc).a[0] = page;
  m.at(loc).a[1] = time;

is making its changes to a copy of frames , not frames itself.

You probably want functions with signatures of the form:

bool pageLoaded(int page, const map<int, pageAndTimestamp>& m)
int findPageToReplace(const map<int, pageAndTimestamp>& m)
void replacePage(int loc, int page, int time, map<int, pageAndTimestamp>& m)
void output(int t, int p, double s, double h, const map<int, pageAndTimestamp>& m)

in which most of the functions take a constant reference, while replacePage requires a (non-const) reference.